1. Field of the Invention
This invention relates generally to the field of wireless communication. In particular, the present invention relates to a wideband antenna having low passive intermodulation attributes for use within a distributed antenna system, the antenna being configured for robust multi-band operation for use in wireless communications.
2. Description of the Related Art
Continued adoption of cellular systems for data transfer and voice communications, along with the introduction of new mobile communications devices, such as tablet devices and the like, make cellular coverage in urban environments an increasing priority. In particular, improving cellular coverage indoors is important to provide a seamless user experience in the mobile communication arena. Distributed antenna systems are becoming increasingly popularized within office buildings and public areas and are used to provide stronger RF signals to improve the communication link for cellular and data services.
A distributed antenna system, or DAS, is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. The idea is to split the transmitted power among several antenna elements, separated in space so as to provide coverage over the same area as a single antenna but with reduced total power and improved reliability. A single antenna radiating at high power is replaced by a group of low-power antennas to cover the same area.
Initial distributed antenna systems were only required to operate over a few frequency bands, resulting in a simplified antenna design process. However, as the communications industry has moved from 2G to 3G cellular systems, and with the advent of 4G communication systems such as Long Term Evolution (LTE), additional frequency bands are required from distributed antenna systems, resulting in more complicated antenna design.
As the density of mobile communication users increases in office buildings and public spaces, and as more users access high data rate features such as file sharing and video downloads, the signal to noise characteristics and RF signal levels of the cellular signals indoors become increasingly important parameters.
To maintain low noise floors in communication systems an important parameter for addressing in the antenna design is Passive Intermodulation (PIM). PIM products are generated when two RF signals at different frequencies are injected into an antenna port; the antenna, though being a passive device, can generate spurious responses due to “natural diode” junctions in the antenna. These natural diode junctions can be formed at the junction of two metal surfaces where the metals are dissimilar. Corrosion and oxidation at these junctions can also cause spurious frequency components due to mixing of the two RF signals. Proper antenna design and material selection is important to meet stringent, low PIM requirements. As PIM components increase, these spurious frequency components add to the noise level, which in turn results in reduced signal to noise ratio of the communication system. This will result in reduced data rates for users.
Thus, modern wireless trends are shaping a need for improved antennas for use within distributed antenna systems, the antennas being capable of wideband operation and low passive modulation attributes.